The invention relates to a method according for controlling a continuously variable transmission of a motor vehicle.
The usual automatic transmission controls with internal combustion engines generally respond to retraction of the gas pedal by reducing the transmission ratio. However, when driving on curves or when braking, this is not always desirable since such load changes can in some circumstances lead to unsafe driving conditions or, when the vehicle accelerates again by increased gas, the transmission ratio must be forcibly increased.
In this connection and in connection with automatically shifting multi-step transmissions, the German Patent document DE-33 41 652 C2 has disclosed that this reduction of the transmission ratio (upshiding) in curves can be avoided by sensing the transverse acceleration of the motor vehicle. However, this procedure can only avoid shifting in curves.
In order to be able specifically to prevent a reduction of the transmission ratio already when approaching a curve, the DE- 39 22 040 A1 corresponding to U.S. Pat. No. 5,025,689, discloses a method for controlling an automatically shifting transmission, according to which the rate of change of the gas pedal is sensed and, if this rate falls below the specified (negative) limit, a signal is generated to prevent upshifting, inasmuch as push-type operation is detected. Upshifting is then prevented, until pull-type operation resumes and a fixed amount of time has passed.
The German Patent document DE- 39 22 051 A1 corresonding to U.S. Pat. No. 5,157,609, additionally provides that this period of time be made dependent on another parameter (driving activity), from a combination of several operating or driving parameters, and which evaluates the driving style of a driver or the currently prevailing traffic situation.
Starting from this prior art, it is an object of the invention to create a method for controlling a continuously variable motor-vehicle transmission, preferably electro-hydraulically operated, which is further improved in particular as regards its behavior in adjusting its transmission ratio or the engine rpm during braking.
According to the invention, this object is achieved by a method for controlling a continuously variable transmission (2) , which is activated electro-hydraulically, of a motor vehicle driven with an internal combustion engine (4), which can be controlled by a power-control element, including one of a gas pedal and a throttle valve, such that the transmission ration (ue) of the transmission (2) is automatically adjusted through the intermediacy of at least one characteristic control curve (RKL.sub.j) in dependence on the position of the power control element (alpha (t)) and the engine rpm (nmot (t)), the method comprising the step of:
controlling the continuously variable transmission by increasing the transmission ratio (ue) at a fifth specific, finite, and slow rate (fifth time derivative of the transmission ratio cg5=due/dt=f(SK(t)), if
a) at least one of 1) a service brake of the motor vehicle is activated (brake signal b=1 and 2) the time derivative of the driving speed (dv(t)/dr) is less than a first negative longitudinal-acceleration limit value (albg (ue, nmot, t), albg(ue, nmot, t)&lt;0 (dv(t)/dt&lt;albg(ue, nmot, t)); and
b) a transverse acceleration (aq(t)), sensed by means of a transverse-acceleration sensor (16), falls below a first specific transverse-acceleration limit curve (aqgl(v(t))) (aq(t)&lt;aqgl(v(t))); and
c) the time derivative of the driving speed (dv(t)/dt) is greater than a second negative longitudinal-acceleration limit value (albbg(nmot, ue, SK(t), t)=k(ue, SK(t)) * albg(ue, nmot, t)&lt;0, (dv(t)/dr&gt;dt albbg(nmott, ue, SK(t)=k(ue, SK(t)) * albg(ue, nmot, t)); and
d) the driving speed (v(t)) is less than a driving-speed limit value (vg(ue, SK(t))), (v(t)&lt;vg(ue, SK(t))).
The advantages of the invention primarily are to be regarded as the circumstance that a method has been created to control a continuously variable motor-vehicle transmission, preferably operated electro-hydraulically, in which the way the transmission ratio is adjusted during braking, especially before curves, has been further improved.
When certain non-critical conditions exist, the transmission ratio is here increased during braking, at a final third speed, such that here, too, the speed of adjustment is less than the maximum speed of adjustment.
Adherence to the conditions in this connection guarantees safe operation of the motor vehicle. In particular, the system monitors that the transverse acceleration is not too high, that the vehicle is not slowed down too much and that the driving speed is not too high, especially so as to avoid a loss of the longitudinal and lateral guiding forces of the wheels of the motor vehicle. When the transmission ratio is increased, the braking torque of the driving (internal combustion) engine can affect the drive wheels more strongly. However, now this cannot have a negative effect on the driving behavior of the motor vehicle.
An increase of the transmission ratio during braking preferably is initiated when a state of a transmission-ratio retention mechanism is active. The latter is activated in well-known fashion when the motor vehicle approaches a curve and the driver no longer presses on the gas pedal.
Increasing the transmission ratio during braking, on the one hand, increases the braking effect of the motor vehicle engine in push-type operation, so that the brake (service brake) of the motor vehicle is relieved of stress. On the other hand, in connection with retaining the transmission ratio before, during and after curves, the optimum transmission and engine rpm is always made available to the driver for accelerating the motor vehicle again after passing a curve.